This invention relates to gas turbine engines and particularly to louver constructed combustor liners.
This invention constitutes an improvement over the liner disclosed and claimed in U.S. Pat. No. 3,643,430 granted to John Emory, Jr. and Joseph Faitani on Feb. 22, 1972 and assigned to the same assignee. As shown in this patent the outer liner is formed from a plurality of louvers suitably attached to form a combustion chamber and an annular cooling chamber. As is well known in the art cooling holes are formed in the upstream vertical wall of each louver so that cooling air is introduced into the combustion zone and the construction of the liner includes a lip portion that directs the air into a film of cooling air. Owing to the extremely high temperature these lips have a tendency of buckling and the art has shown different means intended to prevent the lip from collapsing. Obviously, collapsing of the wall would destroy or impair the film cooling of the downstream louver wall.
One such means, for example, for preventing the lip from total collapsing was to locate a plurality of "dimples" formed in the hot wall of the liner (that wall closest to the combustion chamber) circumferentially spaced about the wall. The "dimple" defining a spacer projected toward the adjacent wall of the downstream louver short of touching. When the lip buckled owing to the high temperature levels, the spacer bore against the adjacent wall and while it blocked the overhead flow, cooling flow between spacers migrated downstream. The "dimple" since it was stamped into the sheet metal louver contained high stress points upon extended operation cracking propagated upstream thereof.
Since the "dimple" acts as a blockage to the cooling flow immediately downstream thereof, and as a chuting passage for hot gases generated by the combustor, severe louver burning was evidenced.
We have found that we can obviate the problems noted above by locating the "dimple" spacer on the cooler wall of the liner (the wall furthest away from the combustion chamber) and locating a hole in or adjacent the "dimple" for directing cooling air to discharge downstream thereof.
In another embodiment this invention comtemplates including cylindrically shaped spacers (posts) such as rods, rivets or the like, projecting either from the cool or hot wall and extending between the walls. The dimension of the posts are such that it minimizes cooling flow blockage and the resultant entrainment of hot gases. Thus the posts are sufficiently spaced from the apertures in the louver that supply the air for cooling such that there is sufficient distance to assure that mixing of the air egressing from adjacent apertures occurs unimpaired to form a substantially circumferentially uniform air film upstream of the posts. Also, the posts are axially spaced from the discharge end of the lip so that the air flowing pass the posts coalesces and reforms a substantially circumferentially uniform air film. It has been found that an efficacious design criteria for minimizing liner burning is as follows: post diameter=0.095 inch; height of louver spacing=0.095 inch; distance from post center line to outer edge of the hot wall 0.095 inch (all within a.+-.0.010 inch tolerance). While these actual dimensions were taken from an actual successfully tested burner liner on a given engine, it can be seen that the relationship of post diameter to height of louvers to post spacing from louver edge is in a 1:1:1 ratio. Also spacing of adjacent louvers is one half inch on center. Both types of spacers, (posts or dimples) terminate short of the adjacent walls when in the unheated condition.